In a mobile communication system of a conventional time division multiple access (TDMA) scheme, different frequencies are used in adjacent cells with each other. A group of frequencies is used for a set of a plurality of cells, and the same group of frequencies is used for another set of cells that is geographically distant. Although this method is preferable from the viewpoint of reducing other cell interference, frequency use efficiency is not necessarily high.
On the other hand, in the W-CDMA (Wideband Code Division Multiple Access) scheme such as IMT-2000, the same frequency is used in every cell by identifying users using spreading codes. This technique is called “one cell frequency repetition”. According to the one cell frequency repetition, the frequency use efficiency and the system capacity largely increase. It is predicted that realizing the one cell frequency repetition is required in the future mobile communication system such as IMT-Advanced (also called LTE-Advanced in 3GPP (3rd Generation Partnership Project)) that will be studied later.
However, since the same frequency is used among the adjacent cells, there is fear that interference level (inter-cell interference) tends to become large particularly at the cell end.
As to inner-cell interference, in the W-CDMA scheme, OVSF (Orthogonal Variable Spreading Factor) code is used in the downlink for realizing inner-cell orthogonalization (that is, inter-user orthogonalization). However, in the W-CDMA scheme, orthogonalization cannot be realized in multipath environment, and uplink is non-orthogonal. By the way, in the E-UTRA (Evolved UMTS Terrestrial Radio Access) scheme, orthogonalization is realized by performing frequency scheduling in the base station for both of the uplink and the downlink.
On the other hand, as to the inter-cell interference, a technique called Inter-Cell Interference Coordination (ICIC) is used in the E-UTRA scheme for realizing inter-cell orthogonalization. In this technique, in addition to that a common frequency is used for every cell, a different frequency is used for each cell at the cell end (refer to non-patent document 1).
FIG. 1 is a diagram showing the inter-cell interference coordination adopted in the E-UTRA scheme. In the inter-cell interference coordination, radio resources are divided into radio resources R1-R3 that can be used only by respective base stations and radio resources R4-R8 that can be commonly used for all base stations. The radio resources R1-R3 are frequencies to be assigned to users at the cell end. The radio resources R4-R8 are frequencies to be assigned to users in an area other than the cell end (for example, users located near the base station). The radio resource R1 is used for users belonging to the cell end of the base station BS1, and the radio resource R1 is not used in adjacent base stations. In the same way, the radio resource R2 is used for users belonging to the cell end of the base station BS2, and the radio resource R2 is not used in adjacent base stations. The radio resource R3 is used for users belonging to the cell end of the base station BS3, and the radio resource R3 is not used in adjacent base stations. Therefore, users at the cell end of the base stations BS1-BS3 can perform communication in a state of small interference.
Information on the radio resources that can be used only by respective base stations is shared among base stations by using backhaul (core network) and/or radio control signal. The inter-cell interference coordination is called autonomous decentralized type inter-cell interference coordination.
In addition, for reducing inter-cell interference, a technique is being studied in which one base station collectively manages radio resources of adjacent base stations. Such an inter-cell interference technique is used for reducing inter-sector interference when the cell of the base station is divided into a plurality of sectors, and for reducing inter-cell interference when there is an overhang cell.
FIG. 2 is a diagram showing inter-cell interference coordination when the base station BS1 collectively manages radio resources of the base stations BS2 and BS3. The base station BS1 is connected to the adjacent base stations BS2 and BS3 using optical fibers and the like. The base station BS1 collectively assigns radio resources to be used by the adjacent base stations BS2 and BS3. For example, the base station BS1 assigns radio resources R1, R2 and R5 to users in the base station BS1, assigns radio resources R3 and R4 to users in the base station BS2, and assigns radio resources R6-R8 to users in the base station BS3.
Accordingly, the base station BS1 can assign radio resources such that interference does not occur. In this example, such inter-cell interference coordination is called centralized control type inter-cell interference coordination. In addition, the base station that collectively manages the radio resources is called a control base station or a centralized control base station, and a base station for which radio resources are managed by the control base station is called a remote base station.
[Non-patent document 1] 3GPP R1-060670, Siemens, “Interference Mitigation by Partial Frequency Reuse”